As the Red Sox take the field for the first time this season at Fenway Park on Monday, a plane circling nearby will drag a banner that reads “Steve Lynch for Oil Evil Empire.”

The attack is not from US Representative Stephen F. Lynch’s Democratic rival or any of the Republicans in the race for the US Senate.

Instead, it comes courtesy of Thomas F. Steyer, a billionaire California hedge-fund executive and Democrat who has chosen to bombard Lynch with theatrical attacks from the skies and from the streets. He has poured $400,000 into the Massachusetts race so far, bankrolling planes with banners, trucks with video screens, and canvassers who plan to knock on 300,000 doors statewide.

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Steyer is hoping to be the left’s answer to the Koch brothers, the billionaires who fund conservative causes, and has seized on Lynch’s Senate campaign against US Representative Edward J. Markey as a test case of his ability to elevate the issue of climate change in the political discourse.

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Last month, he burst onto the scene with a sharply worded threat to Lynch, warning him that if he did not change his mind and oppose the proposed Keystone XL oil pipeline “by high noon on Friday,” he would launch an aggressive campaign to defeat him in the April 30 primary.

Lynch, a former ironworker from South Boston, has said he believes the United States should consider building the pipeline from Canada to the Gulf of Mexico because it could create
construction jobs.

Steyer, like Markey, a favorite of the environmental movement, argues the pipeline will wed the United States to dirty oil and could contaminate ground water if it leaks.

But Steyer’s Wild West rhetoric, and determination to use giant sums of money to sink Lynch because of his beliefs, has stirred criticism from activists who worry about the influence of money in politics.

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“It’s a terrible problem, whether it’s a billionaire or an organization, holding our elected officials hostage to huge amounts of money that can be dropped in their race,” said Pamela Wilmot, executive director of Common Cause Massachusetts, a government watchdog group. “Money should not be allowed to amplify one person’s voice to the extent that it drowns out all others.”

Some environmentalists argue that Steyer’s spending and flamboyant tactics represent a vital counterweight to the influence of big oil companies in elections. They have welcomed Steyer and his so-called super political action committee to the Massachusetts race, saying it is especially needed after the Citizens United decision by the Supreme Court opened the floodgates for corporate spending in politics.

“He’s got money he’s willing to spend to help people understand what’s at stake here, so I say, ‘Hell, yeah,’ ” said Craig Altemose, a Somerville climate activist who encouraged Steyer to get involved in the Senate race. “Let’s use this to help our country solve the greatest problem we’re facing.”

Steyer declined to be interviewed, but has said he is determined to defeat candidates who he believes have not adequately confronted climate change.

“The goal here is to destroy these people. We want a smashing victory,” he told The Hill newspaper last week.

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A Steyer spokesman, Chris Lehane, declined to say how much Steyer plans to spend to defeat Lynch, but said Steyer considers his money a “a drop in the oil bucket” compared to what oil companies spend on lobbying and advertising.

“We see this as a David-versus-Goliath endeavor,” Lehane said. Lynch argues that Steyer is the angry colossus here.

“It seems pretty hypocritical that this guy vows to spend millions against me because I am still considering Keystone, but raises millions for President Obama, who is still considering Keystone,” Lynch said in a statement. “But I guess this is what billionaire Democrats do now — they forget about working people.”

A major Democratic donor who last week cohosted a fund-raiser for President Obama, Steyer, 55, made his fortune as the founder of Farallon Capital Management, an investment firm in San Francisco.

In 2010, he spent more than $30 million to defeat a ballot question in California that would have suspended that state’s global warming law. Last year, he spoke at the Democratic National Convention, advocating for clean energy technologies. He then left Farallon Capital to step up his involvement in politics.

During the last several weeks, Steyer has sent a plane with a banner to fly over Boston before a Bruins game, blasting Lynch for backing “dirty oil.” He plans to send another plane with a banner to the Boston Marathon. Trucks he has dispatched have rolled through Boston displaying videos that accuse Lynch of wanting to export oil to China. Steyer has also bankrolled the League of Conservation Voters’ field campaign against Lynch.

Steyer has not, however, been able to run television ads, unlike most billionaires with a super PAC, because Lynch and Markey have a signed a pledge designed to keep outside groups from running ads or sending mailers in their race.

The campaign has arguably put Markey in an awkward position. He stands to benefit from Steyer’s barrage against his rival. But the image of an out-of-state billionaire hammering a former ironworker could also backfire on Markey, and he has urged Steyer to stay out of Massachusetts.

“Ed Markey has repeatedly repudiated Tom Steyer’s threats and ultimatums in this race and once again calls on him to end his involvement in the special election,” said Giselle Barry, a Markey spokeswoman. “This race should be focused on big issues, not big-money outside interests.”